TomoTherapy Incorporated was founded in 1997 to develop and commercialize a new type of radiation therapy device for improving cancer treatment. TomoTherapy's first FDA cleared product, the HiArt System, combines, for the first time, the means to accurately plan, deliver, verify and review treatments for cancer therapy, and as a result, to greatly improve treatment results and lifestyles of cancer patients because of the better targeting of irradiation to the tumor and the sparing of normal tissue. This "entire radiotherapy department in a machine" integration and automation greatly reduces the need for human intervention, thus reducing staff needs for cancer treatment. The HiArt System employs image-guided radiation delivery, enabling tumors to be irradiated more effectively while saving the critical organs surrounding the tumor. Central to the capability of image-guided radiotherapy is a detector system that efficiently detects high energy x-rays employed in cancer therapy. The objective of the proposed project is to develop a new type of megavoltage detector system that meets all the requirements of tomotherapy imaging applications. The proposed design combines i) amorphous selenium (a-Se) material which has seen extensive research and development in applications ranging from imaging and display, to radiology and astronomy, with ii) TomoTherapy's patented technology for improving detection efficiency for high energy x-rays. This new detector system will improve the detection efficiency, a key requirement for an imaging detector, over the existent technology by an order of magnitude, thus offering great improvement in resolving power on the images. When employed, the multi-row approach and the improved detection efficiency and readout speed of the new detector will improve the tomotherapy imaging quality and throughput significantly, and offers great potential for imaging the patient in real-time during treatment, a key element for further improving cancer treatment where organ motion is a significant obstacle to improved precision. It is expected that the proposed detector will also save $10 to $20 million in tomotherapy manufacturing cost yearly. This technology also has potential applications in other areas as well, including security inspections at ports, forest inspections and orthopedic imaging where there are implanted metal devices or joints.